iSPEX

Scientific power in people's pockets.

iSPEX 2 is a research-grade spectropolarimeter, validated in 2025 for above-water remote-sensing reflectance (Rrs) measurements with Plymouth Marine Laboratory. Our research kits are currently being used in scientific studies for water quality measurements and advanced spectral analysis. We collaborate closely with academic institutions and research laboratories worldwide.

Citizens should not only report environmental change — they should be able to measure it. iSPEX turns an ordinary smartphone into a scientific optical instrument: a small diffraction-grating add-on plus a companion app that calibrates the phone's camera into a real measurement device. That belief — that a phone can be part of the measurement itself, not just a screen between a person and a database — is what Pocket Science has built iSPEX around.

How it started

iSPEX began in 2013, in its original form, as a nationwide Dutch citizen-science campaign to measure atmospheric aerosols (dust particles) with a smartphone add-on. The iSPEX team, led by Frans Snik of Leiden University, had thousands of participants perform measurements throughout the Netherlands on three cloud-free days that year. Each measurement recorded the spectrum and linear polarization of sunlight scattered by suspended dust particles, which the team converted to Aerosol Optical Thickness (AOT) after quality-checking and averaging the submitted data.

The resulting AOT maps matched comparable data from satellites and the AERONET ground station at Cabauw, and in areas with dense enough measurements could resolve detail down to about 2 km — finer than satellite data, and filling blind spots between established ground-based monitoring stations. These first results were published as "Mapping atmospheric aerosols with a citizen science network of smartphone spectropolarimeters" (Snik et al., Geophysical Research Letters, vol. 41, 2014), which credits 3,187 iSPEX citizen scientists as co-authors. A follow-up campaign, iSPEX-EU, ran in multiple European cities in 2015.

That 2013–2015 campaign is historical background, not current activity — iSPEX2 continues the same instrument-and-citizen lineage, now applied to water quality through above-water Rrs measurements. The original campaign record is preserved in the iSPEX archive.

The vision: a distributed citizen observatory

iSPEX is built around a simple idea: validate one instrument concept once, then put it in many hands. The model looks like this:

• One validated instrument concept — the same calibrated optics and processing method, not a bespoke setup per user.
• Many participants — every phone with an iSPEX add-on becomes a measurement point, not just a reporting tool.
• Measurements distributed across space and time — coverage that fixed monitoring stations or research-vessel campaigns cannot achieve alone.
• Meaningful data for researchers — calibrated, citable, comparable to reference instruments, not anecdotal.
• Future potential across water and air — the original iSPEX established an air-quality (aerosol) lineage; iSPEX2 has now proven the above-water Rrs method for water quality. Bringing both into one instrument concept is the long-term ambition.

Imagine thousands of people measuring water or air quality with a calibrated scientific instrument already in their pocket, contributing through a validated method rather than a subjective report. That is the distributed citizen-observatory idea iSPEX is working toward.

What iSPEX actually measures

iSPEX's water-quality method measures above-water remote-sensing reflectance (Rrs): the ratio of light leaving the water surface to the light illuminating it, resolved by wavelength. Rrs is a core quantity in aquatic and water-quality research — it is what turns a phone camera's raw colour data into a calibrated optical quantity that can support algorithms estimating water-quality properties such as algae concentration, sediment, or dissolved organic matter — iSPEX measures Rrs itself, not these properties directly. Deriving Rrs from above the water surface (rather than requiring an in-water sensor) is what makes a handheld, citizen-operated instrument like iSPEX practical at scale.

In practice, iSPEX measures the raw inputs to that calculation — downwelling sky irradiance, sky radiance and water-leaving radiance — through a captured spectrum image, which the processing pipeline then converts into calibrated Rrs values.

We have not published a processed Rrs-vs-wavelength plot here because none has been made publicly available yet — only the raw campaign radiometry. The underlying raw data is open on OSF rather than shown here as a synthetic or illustrative graph.

Scientific lineage & ownership

iSPEX is owned by Pocket Science. Its scientific origins are at Leiden University. Pocket Science developed the iPhone implementation and the surrounding apps, processing, backend and deployment system. Scientific partners retain their own scientific contributions and roles.

FAQ

Downloads

Relevant papers

iSPEX calibration instructions

The iSPEX unit is composed of a backplate, tailored to your phone, and a detachable unit. Two configurations have been developed: a spectrometer version and a polarizer version. Each unit comes with a barcode for support purposes.
To install, remove any cases from your phone (screen protectors can stay) and ensure the camera is clean, as the iSPEX is an optical add-on. Choose the 1x zoom camera (you can find out by positioning the unit and check the screen if it gets dark.

To use with the iSPEX app, position the unit so the horizontal bar, referred to as the slit, aligns within the top box on your screen, making sure both parts of the spectrum are visible. Adjust the exposure if necessary, keeping in mind that overexposed images cannot be used. Press the capture button to take a measurement. The image will automatically be sent to our server for processing. Results, including graphs and data, will be accessible within seconds by pressing the 'MySPEX' button located in the top right corner.

Evidence & resources